A precise phase relationship between a chrominance signal with respect to a predetermined reference carrier signal is required under various conditions. For example, a video signal, derived from a magnetic tape apparatus, may have to be synchronized, in phase, with a local reference, for example a clock pulse in studio apparatus in order to permit mixing and phasing-over of recorded signals with signals derived in the studio. Time base errors may occur due to inaccuracies in the running of the magnetic tape in the magnetic tape apparatus. These time base errors result in phase errors in the chrominance signal, which, when reproduced, become apparent as undesirable changes in color saturation and hue. Reproduction of a video signal from magnetic tape under slow-motion conditions, or as a stopped image, also requires reintroduction of corrected chroma carrier phase signals with respect to a studio chroma carrier signal of studio phase. Other signals may require phase synchronization, and rather than signals derived from magnetic tape apparatus, external signals which are received, for example, over beamed paths, from satellites, or the like, may be conducted into a studio for processing therein.
It has previously been proposed--see, for example, the referenced German Patent Disclosure Document No. DE-OS 22 40 816 (claiming priority U.S. Ser. No. 172,982, of Aug. 19, 1971), Mesak, to correct chroma carrier phase errors by controlled delay lines. This permits synchronization of any asynchronously coded video signal, transmitted by, for example, the NTSC or PAL system, with a predetermined reference. The apparatus required is complex and needs many components due to the extensive memory capacity which is needed, in order to store video images. The synchronization of the audio signal is lost.
The referenced published article by H. Fix in the publication "RTM", No. 6, 1968, pp. 249-259, describes a system in which two set voltages are derived corresponding to phase deviation of the chroma signal from a reference carrier signal; the so-derived set voltages are stored for the duration of a line scan (see particularly pages 257, 258 of the cited literature). The chroma synchronizing signal is separated from the chrominance signal and applied to a phase demodulator which has two synchronous demodulators. The output derived from the phase demodulator will be pulses of the demodulated chroma synchronization signals, the amplitudes of which will be a measure of the phase difference between the reference signal and the chrominance signal derived, for example, from a recording apparatus. This is the chrominance signal which may have an incorrect or erroneous phase position. The voltage values of the pulses are stored in a memory for the duration of a line scan. The resulting step voltages are transformed back by two synchronous modulators in a chrominance carrier oscillation. The signal then will have the same phase as the chrominance synchronization signal derived from the apparatus, the oscillation, however, being available during the entire line scan.
The apparatus required is complex, since two demodulation processes are necessary for the chroma difference signals. These signals, on the one hand, are demodulated by means of the studio reference signal and, on the other, by means of the chroma carrier from the input signal--which, as above described, may have the erroneous chroma carrier phase position. An additional difference formation of the two demodulation products is necessary, so that, to generate a correction signal, an additional modulation of the reference signal with the signal components of video frequency must be carried out.